The invention relates generally to a temporary orthopaedic plate device used for fixation and stabilization of one or more bone fragments with a means to secure and lock the fixation fastener to the plate to prevent the fixation fastener from backing out of the bone. In particular, the device is a fixation plate that contains a dovetail groove along the longitudinal axis that can accept a sliding lock plate with a dovetail shaped cross section. Initially the lock plate is secured (or tethered) to the fixation plate so that it has a limited amount of relative movement but together the two plates form a unit.
The dovetail shaped sliding lock plate contains a slot, referred to herein as a lock slot. This slot includes a conical shaped countersink located coaxially at one end of the slot. The lock slot allows a conical headed screw to pass through it and to be inserted into a threaded recess of the fixation plate to secure the sliding lock plate and limit its movement from one end of the lock slot to the other end of the lock slot. The sliding lock plate also contains cutouts and through holes to allow fixation fasteners to be placed through and into the holes in the fixation plate and in turn into the bone or graft material.
Sliding the lock plate to one end of the lock slot allows the cutouts and through holes to align with the fixation holes in the fixation plate. Sliding the lock plate to the other end of the lock slot allows the edges of the sliding lock plate to overlap over the holes in the fixation plate to secure the fixation screws and prevent the heads of fixation screws from backing out of the plate construct. It is when the sliding lock plate is slid to this position that the conical shaped countersink in the sliding lock and the conical headed lock screw align to cam the lock plate into position so as to lock down or fix the sliding lock plate into position in the fixation plate. Thus, the lock plate no longer slides relative to the fixation plate.
In a further embodiment of the invention, instead of using a lock screw to secure the lock plate into position, a spring biasing member is formed in the plate by forming a cut-out. The spring biasing member includes a boss which is held in a divot in the stabilization plate when the lock plate is in a first or xe2x80x9copenxe2x80x9d position so that the screws can be implanted. This divot is shallow enough to allow the plate to be easily pushed into the xe2x80x9cclosedxe2x80x9d position where the boss encounters a deeper lock hole, which locks the plate, and thereby also the screws into position. The head of the bone anchor screws have a special step cut around the top to allow room for the lock plate without requiring the screws to be driven to an exact depth.
Orthopaedic implants have evolved into many types of devices to assist in arthrodesis and correction of bone defects of a congenital, degenerative, or trauma related nature. Among the various types of orthopaedic implants are plate type devices. Plate type devices, like most devices, with the exception of endoprotheses, are temporary devices attached to stabilize two bone fragments or two bones, such as vertebra, until healing of the fragments or fusion of the two bones has occurred.
These devices are designed to be load sharing rather than load bearing. Load bearing devices typically carry all or bear all the stress. This is sometimes referred to as xe2x80x9cstress shieldingxe2x80x9d. Load sharing transfers some amount of the stress from the device to the bone itself. This transfer of load to the bone causes stress and this stress becomes the mechanism that triggers the body to start the healing or fusion process.
Some applications require different types of fastener devices, such as screws, pins, staples, or cerclage wire, in conjunction with the plate devices to secure them to the bone to provide the required stabilization. Many fasteners are designed specifically for the two different types of bone within the body.
The two types of bone are cortical and cancellous bone. Cortical bone is typically the hard, dense shell of the bone that provides the structural strength. Cancellous bone is the more spongy and soft bone located inside the cortical shell as part of the marrow of the bone, which provides the blood supply and nutrients for the bone.
Due to the hard, dense shell cortical bone is typically more stable for the placement of screws for fixation. Cancellous bone is weaker compared to cortical bone. Screws designed for fixation in cortical bone are typically placed through one cortex, through the cancellous or marrow, and into the far cortex of the bone. This is referred to as bicortical screw fixation. Screws designed for cancellous bone are typically designed with a buttress type thread to be able to put as much material as possible between successive threads to increase the shear area in the cancellous bone. Cancellous screws are typically placed through one cortical wall and sized in length such that the end or tip of the screw does not encounter the cortex on the far side of the bone but ends in the cancellous structure. Cancellous screws may be used instead of cortical screws when penetrating the far cortex is not preferable. In some cases, penetrating the far cortex may result in damage to arterial or neurological structures. However, one concern of screws placed unicortically into cancellous bone can be the tendency for the screw to xe2x80x9cback outxe2x80x9d from the plate device under cyclic loading and/or osteoporotic conditions or due to poor quality of the bone.
Devices used in applications involving the fusion or arthrodesis of two bones, such as the vertebra, require the cartilaginous material to be removed between them and the bone surface abraided to encourage a bleeding surface. Blood supply from the bleeding surfaces are required in order for the bone to fuse. Fusion of a joint involves removing the cartilaginous material in the joint and requires the cartilaginous surface of the articular joint to be abraided to encourage a bleeding surface for fusion. Fixation and stabilization must be adequate for the time required for a fracture to heal or two bones to fuse.
In certain applications where the devices are used in close proximity to a joint, the device should be designed such that it does not cause damage or have adverse effects to the articular surfaces of the joint. Further considerations of implant design should also be given to ensure that ligaments and tendon structures, usually located close to the joint, that come into contact with the implant are not compromised in any way by excess material, rough surfaces, or sharp edges. The profile of the present plate construct has been designed with these considerations.
In one embodiment, an anterior cervical plate assembly is provided with a fixation plate having exterior flanges each having a spherical counter sunk through hole to receive a cancellous screw for fixation. Recessed cutouts between these flanges provide for an increased ability to view the implantation site during fixation. These cutouts also allow easier bending by reducing the cross section of the plate. Further, the plate may include an additional aperture for fixation of a graft screw in the event a graft is used with the invention. Additionally, the top surface of the plate includes a channel defined by opposing undercut flanges, which form a sliding dovetail connection with mating edges of a locking plate. The locking plate further includes openings located to correspond and give access to the cancellous screw openings of the fixation plate when the locking plate is in a first position. This position is defined by a lock slot which receives a swaged lock screw, secured in the fixation plate. The locking plate can be slid to a second position where it does not overhang the fixation plate, and which is defined by the other end of the lock slot. In this position, the lock screw encounters a counter sink so that it can be tightened into a flush position relative to the top of the locking plate. The screw head includes a corresponding bevel to bias the locking plate into the second position in the countersink of the locking plate. Further in the second position of the locking plate, the bottom surfaces surrounding the edges of the cancellous-screw apertures now press against the top surfaces of the fixation plate surrounding the cancellous screws apertures. This blocks the heads of the cancellous screw from backing out of their apertures and locks them into position in the plate. The graft screw is, likewise, locked into place by the single sliding motion of the locking plate.
In a further embodiment of the invention, the locking plate has a leif spring formed in it by cutting a u-shape in the central portion of the plate. The spring has a boss on its bottom side which interfaces with a lock hole in the plate to lock it into the closed position, and with a shallower divot to hold the lock plate in an open position. The screws also have a step cut to allow more clearance for the plate.